The ring acylation of phenols is almost impossible to achieve in good yields. Suitable methods for the C-acylation of phenols of the resorcinol and phloroglucinol type do not exist in a general sense. As aptly pointed out by E. Berliner, "Organic Reactions," Vol. V, pages 236-7, John Wiley & Sons Inc., New York, 1949, direct acylation of phenols by a Friedel-Crafts procedure is not particularly advantageous, because complex mixtures of C-acylated and O-acylated compounds are generally obtained. The Houben-Hoesch reaction, which is described by the authors in Ber. 48, 1122, 1915 and Ber. 59, 2878, 1926, runs fairly smoothly, particularly with polyphenols, but has considerable disadvantages as well, e.g., the use of nitriles which lead to the formation of appreciable amounts of by-product iminoethers. The Fries rearrangement, described in detail by A. H. Blatt, "Organic Reactions," Vol. 1, 342, John Wiley & Sons Inc., New York, 1942, is a good method for obtaining o-acyl and p-acylphenols, generally as a mixture of the two products, in which the proportions of both isomers depend on reaction conditions, but this rearrangement is practically of minor importance if C-acylated polyphenols, more particularly if C-acylated polyphenols substituted in the ring with other groups, are desired.
A suitable method for preparing C-acylated polyphenols involves first protecting the hydroxy groups by transforming them into methoxy groups by reaction with dimethyl sulfate or diazomethane, as reported by Saul Patai, "The Chemistry of the Hydroxyl Group," Part 2, page 1006, John Wiley & Sons, 1971, subsequent acylation being carried out according to known procedures, and subsequent restoration of the free hydroxy groups by a usual demethylation process. Because of the number of steps which are involved, this method is quite tedious and cumbersome. Moreover, complete demethylation is difficult to achieve.